1. Field of the Invention
The present invention relates to an electric fan and more specifically, to fan bearing structure in which the stepped hub has an inside annular flange for the coupling of the axle of the fan blade assembly to keep the axle in coincidence with the central axis of the axle hole of the stepped hub of the bearing block for smooth and stable rotation without noises.
2. Description of the Related Art
Following fast development of high technology, electronic devices are created having the characteristic of small size and high performance. In these electronic devices, small electronic components are installed in a limited area. In consequence, a big amount of heat energy is produced during the operation of a high-performance electronic device. Excessive high temperature may cause electronic ionization and thermal stress, resulting in low stability of the electronic device. Therefore, it is important to dissipate heat during the operation of an electronic device. Normally, a cooling fan and a heat sink are directly installed in an electronic device for quick dissipation of heat.
Therefore, using a fan to dissipate heat from, for example, a CPU has become a requisite measure. A fan general uses a bearing to support the axle of the fan blade assembly, allowing rotation of the fan blade assembly at a high speed. Therefore, a bearing for this purpose must have the features of long working life, low noise level, low power loss and excellent heat dissipation effect. To achieve these objects, an excellent lubrication effect between the bearing and the axle of the fan blade assembly is needed. Excellent lubrication effect between the bearing and the axle of the fan blade assembly lowers friction during rotation of the axle relative to the bearing. Therefore, lubricating oil is commonly used and applied to the bearing and axle of a fan. However, lubricating oil may leak out of the bearing or evaporate if the fan is turned upside down or kept in a horizontal position during packing or delivery of the fan. Further, dust or impurities may enter a gap between the bearing and the axle, causing friction between the bearing and the axle during rotation of the axle. Friction between the bearing and the axle during rotation of the axle results in noises and vibration of the fan blade assembly.
FIGS. 8 and 9 show a fan bearing structure according to the prior art. According to this design, the bearing A has a body A1, an expanded base A2 at the bottom side of the body A1, an axle hole A3 at the center of the body A1, and a plurality of hooks A4 at the top side of the body A1 around the axle hole A3. The hooks A4 each have a protruding portion A5. After insertion of the axle B1 of the fan blade assembly B into the axle hole A3, the protruding portions A5 of the hooks A4 are forced by the springy power of the material of the hooks A4 into engagement with the annular groove B2 around the periphery of the axle B1, thereby holding the axle B1 to the bearing A and allowing rotation of the fan blade assembly B relative to the bearing A.
The aforesaid fan bearing structure is still not satisfactory in function because of the following drawbacks:    1. The bearing A must have a certain height for receiving the axle B1 and supporting rotation of the axle B1. The design of the hooks A4 further increases the height of the bearing A. To fit the height of the bearing A, the length of the axle B1 is relatively increased. Thus, the fan blade assembly B cannot have a low profile for use in an electronic device having light, thin and small characteristics.    2. Because the hooks A4 are disposed at the top side of the body A1 around the axle hole A3 and have the respective protruding portions A5 suspending in the axle hole A3, the hooks A4 may be permanently deformed or damaged accidentally during insertion of the axle B1 into the axle hole A3, and the axle B1 may be jammed in the axle hole A3 of the bearing A in case the hooks A4 are deformed or damaged.    3. The hooks A4 have a thin wall of low strength. The clamping force of the hooks A4 is low. During high-speed rotation of the axle B1, the axle B1 may escape from the constraint of the hooks A4 and may be unable to keep the axle B1 in vertical, causing vertical displacement of the fan blade assembly B relative to the bearing A or damage of the hooks A4.    4. Because the hooks A4 have a thin wall of low strength and provide a small clamping force. During high-speed rotation of the axle B1, the hooks A4 may be unable to keep the axle B1 in vertical, causing vibration of the fan blade assembly B, and vibration of the fan blade assembly B may damage the hooks A4.    5. The hooks A4 are disposed at the top side of the body A1 and spaced from one another around the axle hole A3. In case the hooks A4 are not accurately positioned or deformed accidentally, the axle B1 may rub against the hooks A4 during its high-speed rotation, causing noises or damage of the hooks A4.
Therefore, it is desirable to provide a fan bearing structure that eliminates the aforesaid problem.